(1) Field of the Invention
This invention relates to head-up display stytems, and concerns in particular ways of highlighting part of a scene as viewed via a head-up display.
(2) Description of the Prior Art
A head-up display -- commonly referred to by the acronym HUD -- is a display that can be seen by the observer for whom it is intended while at the same time he has his head up, looking at some other scene. HUDs are most generally provided to give information to drivers of vehicles --and particularly to pilots of aircraft --in such a way that they can look out of their vehicle at the scene in front of it and yet at the same time have displayed in front of them (appearing to be sufficiently far away so that they need not refocus their eyes to see it) such additional information, in pictorial or alphanumeric form, as they may need. This information may, for example, describe the present state of their vehicle and the range to some point of interest. A HUD in an aircraft, for instance, may be arranged to enable the pilot to see in the same place both the outside scene (a target) and vital instrument readings (height, speed, altitude), so relieving him of the necessity of looking down at his cockpit instruments (which are, by contrast, head-down displays, HDDs).
In its simplest form a HUD is little more than a transparent screen placed in front of the observer and through which the observer can see the real world, one surface of this screen forming a reflective interface, the screen being so disposed that this reflective interface may be used to reflect light from some appropriate source towards the observer's eyes along a path such that to the observer this light appears to originate in the real world he is viewing. Because the HUD screen "combines" two optical images, it is generally referred to as an "optical combiner". However, the acronym HUD, and its parent expression (head-up display), are also used to mean rather more than just the optical combiner; the two terms can also include the complex optical and image-forming systems needed to generate the required information and present it, via the optical combiner itself, to the observer. Nevertheless, for the purpose of this specification the two terms are used to denote a display system comprising a transparent screen (the optical combiner) which is in use positioned in front of the intended observer such that the observer can obtain, through the screen, a view of the scene beyond the screen (usually of the terrain outside and in front of the vehicle in which the display system is mounted), and can obtain, reflected off the screen, a view of a second scene which is itself usually a display of pictorial and/or alphanumeric informational matter. The vehicle is most commonly an aircraft, the observer the pilot, and the HUD screen is normally so arranged that the pilot views at least part of the scene towards which the aircraft is flying through the screen (which screen sometimes may be swung down or otherwise moved from a stowed position to an operative position in front of the pilot, or vice versa, as required). For the most part the following description relates to such an aircraft-mounted HUD.
The information displayed on the HUD screen is quite often alphanumeric in form, being numbers or symbols relating to the flight of the aircraft (such as its altitude, its speed, its compass heading and other data required by the pilot). In addition, however, the information may be pictorial, and may in particular be a true pictorial representation of the real world outside the aircraft. For example, at night, by using a low light level television system, there may be generated and displayed via the HUD screen a picture corresponding to the real scene in front of the aircraft, so that the pilot is provided with a picture which is identical to that scene which he would be able to see through the HUD screen were it not dark.
As will be appreciated, HUD systems find particular application in military aircraft, specifically fighter aircraft, and in one aspect the present invention seeks to provide an improved HUD system, especially suited for use in fighter aircraft, in which targets of particular interest are more readily distinguished.
In U.S. Pat. No. 4,118,733 there is described a surveillance arrangement suitable for use in an aircraft wherein a television camera/display combination provides an image of the viewed scene, and infra-red detector means viewing part of the same scene provides brighten-up signals to effect a localised increase in intensity of the displayed image to highlight heat emitting targets in that part of the viewed scene. In one described embodiment, the output of the infra-red detection means is mixed with the camera signals so that the former acts to brighten up areas of the displayed image generated by the latter which correspond to the positions of heat-emitting targets, and in a development of that embodiment the resultant display is ultimately made available to the pilot via a HUD screen. In this earlier invention the brightened up areas may constitute either a true image of the target or a symbolic representation, or cue, of the existence of the target at that position.
There are, however, a number of occasions when the arrangement of the earlier invention is not satisfactory. Thus, for example, it is sometimes convenient for the visual representation of flight information and target data to be generated not by a television-type display but instead by a source--such as a diode matrix--capable of emitting light of a much higher intensity, so that it may be the more clearly visible under daylight viewing conditions. Additionally, it may well be that there is no need, say, for the pilot to fly his aircraft by sight based upon a television-type display of the real world (as generated by, for instance, a low light level system or an infra-red system), but that target information is still required. In either case it is clearly not possible for target highlighting signals to be mixed with scene-viewing camera signals.
The present invention seeks to deal with this problem by arranging for a target-emphasising visual effect to be generated and projected onto the HUD screen quite separately of the view of the scene, whether that view is obtained directly (through the HUD screen) or indirectly (from an image projected onto the HUD screen).